1. Field of the Invention
The present invention relates generally to materials and methods applicable to brazing sheet materials and in particular to aluminum alloys that maintain strength at high temperatures such as those that are required for automotive, vehicular, or industrial heat exchangers, and specifically in charge air coolers, as well as methods for their manufacture and use.
2. Description of Related Art
Strength enhancement of aluminum brazing sheet at room and elevated temperatures is desirable towards downgauging for weight saving and higher operating temperatures in various heat exchanger applications. Aluminum brazing sheet typically includes a core alloy of 3xxx and a lower melting braze clad of 4xxx series. 3xxx and 4xxx are designations as set forth by The Aluminum Association. As 3xxx series aluminum alloys are generally non-heat treatable, solid solution strengthening is the primary source of enhancing as-brazed strength. However, precipitation hardening to some extent is possible if magnesium is present in the core alloy. Age hardening in 3005 based brazing sheets was demonstrated in earlier work. (See for example, N. D. A. Kooij, J. A. H. Sxc3x6ntgerath, A. Bxc3xcrger, K. Vieregge, A. Haszler, xe2x80x9cNew High Strength Alloys for Brazing with Long Life Corrosion Propertiesxe2x80x9d, VTMS Conf. Proc., Indianapolis, Ind. 971882 (1997); N. D. A. Kooij, J. A. H. Sxc3x6ntgerath, A. Bxc3xcrger, K. Vieregge, A. Haszler, xe2x80x9cThe Development of Two High Strength Aluminum Brazing Sheet Alloys with Long Life Corrosion Propertiesxe2x80x9d, Alumitech Conf. Proc., Atlanta, Ga. (1997) p. 185-190; WO 99/55925 from Hoogovens Aluminium Walzprodukte xe2x80x9cAluminium Alloy for Use in a Brazed Assemblyxe2x80x9d; and H. Scott Goodrich and G. S. Murty, xe2x80x9cAge hardening effects in 3xxx series brazing sheet core alloysxe2x80x9d, VTMS 4 Conf. Proc., I Mech E 1999, London, p. 483, which are incorporated herein by reference in their entireties).
One mechanism of age hardening involves (i) diffusion of silicon from the braze clad into the core alloy, (ii) retention of silicon and magnesium in solution during cooling from the braze cycle, and (iii) then the precipitation of Mg2Si during subsequent vehicle operation or post-braze aging treatment. However, 3xxx alloys are generally not heat treatable, and the primary strengthening mechanism is by solid solution strengthening. It has been reported that age hardening of materials that included Mg in the core material such as 3005 was possible.
In a published study, Goodrich et al., xe2x80x9cAge hardening effects in 3xxx series brazing sheet core alloysxe2x80x9d, VTMS 4 Conf. Proc., I Mech E 1999, London, p.483, which is incorporated herein by reference in its entirety, the aging response of different brazing sheets was monitored through room temperature tensile tests performed immediately after brazing and after aging for various times at 104xc2x0 C., 150xc2x0 C., 175xc2x0 C. and 200xc2x0 C. As the actual heat exchanger operating temperature is generally higher than room temperature, the material properties at elevated temperatures are of interest from a design standpoint. This especially applies to charge air coolers which are used in turbocharged engines and in diesel engines to cool the intake air compressed by the turbocharger prior to its injection into the cylinder chamber. This is done by employing an air-to-air heat exchanger (known in the automobile and truck industry as an inter-cooler or charge-air-cooler). The cooled and compressed air results in maximum performance derived from turbocharging, which lowers emission levels and improves fuel efficiencies. Charge air coolers are exposed to extreme temperature fluctuations and elevations in use, and as such, their construction in terms of metallurgy has proven difficult to design. Standard 3xxx series alloy such as 3003 alloys have been used in the past in some heat exchanger applications since they are easily formed into sheet, fins and tubes. However, they have relatively low strength and generally cannot be used in applications that require subjection to high temperatures. Different aluminum alloys have been used to construct charge air coolers including 3003, but the 3xxx series alloys, while being brazeable, are generally too soft to adequately machine or have the necessary mechanical properties at high temperatures to be acceptable for use in applications such as in charge air coolers. Moreover, the materials must be able to exhibit sufficient strength after long-term exposure to temperatures greater than about 177xc2x0 C. Many vehicle manufacturers have turned to copper and brass for designing materials useful in the manufacture of charge air coolers since these materials can operate at the temperatures required for a charge air cooler (i.e. up to peak temperatures of about 250-300xc2x0 C.). However, copper and brass are much heavier and costlier than aluminum, and as a result, typically cost more to operate in terms of fuel efficiency given their added weight. Thus it would be desirable to have an aluminum alloy that is suitable for use under the conditions required for charge air coolers without using copper/brass alloys. Since the use of turbocharged engines is increasing in both automobiles and trucks, there have been increasing demands on aluminum alloy suppliers to obtain a material that has good formability and acceptable strength over the complete temperature profile that is required for operating a charge air cooler.
An example of additional strengthening at elevated temperatures in charge air cooler alloys was reported by Raybould and Capriotti LaSalle in U.S. Pat. No. 5,857,266, xe2x80x9cHeat Exchanger Having Aluminum Alloy Parts Exhibiting High Strength at Elevated Temperatures,xe2x80x9d the content of which is incorporated herein by reference in its entirety.
In accordance with these and other objects, the present invention provides aluminum alloy brazing sheet materials that have an increase in yield strength when the materials have been exposed to high temperatures for extended periods of time (i.e. temperatures of from 100-250xc2x0 C. or 260xc2x0 C., for times up to 2500 hours) greater than would have been expected. Such increases in yield strength of the inventive brazing sheet materials may be obtained by subjecting the aluminum alloy to a post braze aging treatment. The increase in yield strength of the core materials is unexpectedly retained, even after the materials are subjected to high temperatures for an extended period of time or when the materials are exposed to very high temperatures that would be found in the case of heat exchanger components, particularly in heat exchanger applications where down-gauging would be advantageous such as for header material and most particularly charge air coolers.
In further accordance with the present invention, there is provided a brazing sheet material comprising a core in 3xxx series alloy that includes at least one dispersoid forming element in an amount sufficient to increase the yield strength of the brazing sheet by up to about 20% in a peak age temper, as compared to the alloy in an as-brazed temper.
In further accordance with the present invention, there is provided a brazing sheet material comprising a core in a 3xxx series alloy of the following preferred composition (in weight % based on the weight of the core alloy): Si less than 0.2%, Fe less than 0.2%, Cu: 0.3%-0.7%, Mn: 1.3%-1.7%, Mg: 0.4%-0.8%, Ti less than 0.10% and at least one element selected from the group consisting of Cr (preferably 0.05%-0.20%), Sc (preferably 0.05%-0.20%), V (preferably 0.05%-0.20%), Zr (preferably 0.05%-0.20%), Hf (preferably 0.05%-0.20%) Ni (preferably 0.20%-1%), balance aluminum and unavoidable impurities. In preferred embodiments, the inventive brazing sheet material has been subjected to a treatment sufficient to maintain an increase in strength over a wide temperature range.
Additional objects, features and advantages of the invention will be set forth in the description which follows, and in part, will be obvious from the description, or may be learned by practice of the invention. The objects, features and advantages of the invention may be realized and obtained by means of the instrumentalities and combination particularly pointed out in the appended claims.